to center the butterfly: losen the 2 screws that hold it to the shaft just a tiny bit, unscrew the level setting screw until is not touching, open the throttle and then let it close and push with your finger on the higher edge of the butterfly (highest point as it sits at an angle), check in the light so it's not off center (should have same amount of light on both sides if any, should be almost no light at with some around the shaft), tighten the screws. At this point the butterfly will bind, lift the setting screw until it pushes it up a bit so it won`t bind, that`s your minimum setting and go from there.